Methylation and other types of modification of tRNA and r-RNA in human cells during senescence. Cultured human cells will be used to study; 1. Differenes in modified nucleotide content of tRNA isolated from young and aged human cells. 2. Differences in activities of tRNA modification enzymes in young and old human cells. 3. Differences in isoacceptor species of tRNAs during senescence and 4. If there are aging associated differences in tRNA, the location of these differences is in the tRNA molecules.